This invention relates to filler materials for dental composites and to dental composites containing such filler materials.
So-called amalgam dental fillings are being increasingly replaced with dental composites that more closely match the color and appearance of the natural tooth. These composites generally consist of an organic resin that contains a microparticle filler. Most systems incorporate a light- or UV-curable polymeric resin, such as a diglycidylmethacrylate of bisphenol A (BIS-GMA), triethyleneglycol dimethacrylate (TEGDMA) or a urethane dimethacrylate (UDMA). The filler particles are typically barium silicate glass, quartz or zirconium silicate, combined with small colloidal silica particles.
To match the appearance of the original tooth enamel and provide a natural, tooth-like appearance, the inorganic particles must be translucent and display minimal light scattering. The composite of inorganic particles and polymeric resin must have a refractive index in the range of about 1.48 to 1.60, especially ˜1.54. Both the resin and the filler particles should have very nearly the same refractive index, to minimize light scattering. Polymeric resins can be blended to achieve this refractive index. Obtaining the necessary refractive index for the filler particles is more problematic.
One approach to producing filler particles of the required refractive index is described in U.S. Pat. No. 4,503,169 to Randklev. Randklev produces microparticles containing amorphous silica microregions interspersed with polycrystalline ceramic metal oxide microregions. Silica (SiO2) is a very low cost inorganic material having a refractive index of ˜1.46. The ceramic metal oxide used in Randklev's process is preferably zirconia (ZrO2), which has a refractive index of about 2.20, because ZrO2 has the added benefit of being opaque to x-ray radiation. Microparticles having any refractive index between that of silica and that of zirconia can be produced in Randklev's process by varying the relative proportions of these components, provided that crystalline microregions having diameters of greater than about 400 microns (the shortest wavelength of visible light) are excluded. In Randklev's process, ZrO2 is mixed with SiO2 using sol-gel techniques to form the microparticle composites. One of the problems with the sol-gel process is that the starting materials must be blended carefully and then temperature processed to obtain the composite microparticles. This processing is variable and can lead to voids in the composite microparticle. These voids scatter light and increase the opacity. Large crystallites of ZrO2 that form during the thermal processing also lead to light scattering. This seriously degrades the appearance of the composite. In addition, there is very little control over the resulting refractive index of the dental composite when using these composite microparticles.
It would therefore be desired to produce a filler material for dental composites, which has a carefully controlled refractive index and exhibits a desirable opacity and minimal light scattering.